The present invention relates to a zoom lens that has its full length remaining constant during zooming, and focusing. Specifically, the invention relates to a small-format, wide-angle, high-zooming zoom lens that is well fit for an interchangeable lens system and capable of taking moving images. More specifically, the invention relates to an imaging apparatus equipped with a zoom lens having its full length remaining constant during zooming, and focusing.
So far, some digital camera products have been known to be capable of taking moving images. In recent years, interchangeable type digital cameras, inter alia, an interchangeable lens compatible with the function of the camera body of taking moving images have also been under study.
Only the requirement for conventional cameras designed to take still images has been that they are in focus on target subjects in the moment of image shooting after determination of the composition, because their object has been not to miss blink-of-an-eye shutter chances. For instance, the so-called phase contrast type autofocus (AF) function has been adopted because of its combined high speed/accuracy.
In the taking of moving images with some video cameras designed for professional use, on the other hand, a skilled cameraman has implemented focusing operation in the manual focus (MF) mode. For many consumer-oriented video cameras, however, it is required to put the AF system constantly in operation thereby keeping an in-focus state depending on object distances. To this end, the contrast AF mode (the so-called climbing mode) relying upon an imaging device has been adopted.
Further, to keep the in-focus state, the focus lens is constantly subjected to back-and-fore, fine movement (called wobbling) in the optical axis direction to measure contrast changes. As the in-focus state is judged as being changing, it causes the focus lens to be properly moved for re-focusing operation. This wobbling function requires very fast operation depending on frame rates; so there is still mounting demand for reducing the weight of a lens used for wobbling.
The range of movement of the constantly moving focus lens is kept within the depth of focus. In other words, it is controlled such that any out-of-focus state during wobbling cannot be recognized; however, when there is a large image magnification change (changes in the size of a subject on the imaging plane), images look quite unnatural because they look as if they were always waving. Accordingly, it is still a vital requirement to reduce or minimize the magnification changes during wobbling.
Some products capable of taking moving images have also been known to have a sound recording feature. As there are large noises produced out of the interchangeable lens per se upon zooming operation, it stands in the way to sound recording. For a zoom lens compatible with the function of taking moving images, therefore, it is now required to decrease the groups that keep moving during zooming operation (lens groups and aperture stops) in number so as to place noises stemming from zooming operation down to a sufficiently low level.
For an interchangeable lens used for both the taking of still images and the taking of moving images, it is required to have a wider-angle, higher-zooming feature in association with extended use of digital cameras.
Further, in a recently developed lens-interchangeable digital camera, its camera body is getting smaller and smaller in association with mirrorless design.
With camera body's size reduction and extended use of digital cameras, there is mounting demand for interchangeable lenses to get smaller and have a wider-angle feature and a higher zoom ratio.
So far, a positive-negative-positive-negative-positive five-groups type zoom lens has been known as the zoom lens of the type well fit for a wider-angle, higher-zooming feature.
Patent Publications 1 to 6 have come up with design examples of the positive-negative-positive-negative-positive five-groups type zoom lens suitable for such a wider-angle, higher-zooming feature.
Patent Publication 1 shows that upon zooming from the wide-angle end to the telephoto end, the first to fifth lens groups are designed to move. Patent Publications 2 to 6 show that upon zooming from the wide-angle end to the telephoto end, the first and fifth lens groups are designed to remain fixed. Patent Publications 2, 3, 4 and 6 show that the aperture stop is designed to remain fixed too. Patent Publications 2, 3, 5 and 6 show an arrangement suitable for size reduction wherein focusing is implemented as by the movement of the fourth lens group or the like.